1. Field
Exemplary embodiments relate to a secondary lithium battery electrolyte and a secondary lithium battery including the secondary lithium battery electrolyte that can improve the cycle and high temperature retention characteristics of a battery using the secondary lithium battery electrolyte.
2. Description of the Related Art
As portable electronic devices, such as video cameras, cellular phones, notebook computers, and the like, become more lightweight and have higher performance, more research into batteries used as power supplies for such portable devices is being conducted. For example, chargeable lithium secondary batteries can be rapidly charged and have three times the energy density per unit weight than conventional lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, and the like. Therefore, research and development of chargeable lithium secondary batteries is being actively conducted.
In general, a secondary lithium battery includes a cathode, an anode, a separator disposed between the cathode and the anode, and an electrolyte. The electrolyte is obtained by dissolving an appropriate amount of lithium salt in an organic solvent. The lithium salt may be, for example, LiPF6, LiBF4, LiClO4, LiN(C2F5SO3)2, or the like, and provides a source of lithium ions in the secondary lithium battery, and thus a basic operation of the secondary lithium battery can be performed. In addition, if a carbonate-based, polar, nonaqueous solvent is used as the organic solvent, when the secondary lithium battery is initially charged, a carbonate-based electrolytic solution is partially oxidized on the cathode of the secondary lithium battery. Thus, a passivation layer is formed on a surface of the cathode by such irreversible reaction. The passivation layer is referred to as a solid electrolyte interface (SEI) membrane. The SEI membrane prevents the electrolytic solution from being oxidized any more, and allows the secondary lithium battery to retain stable charge/discharge capacity. In addition, since the SEI membrane functions as an ion tunnel so as to allow only lithium ions therethrough and to prevent a large excess of lithium ions from being eluted, a cathode structure can be prevented from collapsing.
However, when the SEI membrane is excessively thick, the electrolyte is depleted in the secondary lithium battery, and thus the performance of the secondary lithium battery may decrease. In addition, when the secondary lithium battery is initially charged, since a carbonate-based organic solvent is decomposed during the forming of the SEI membrane, a gas is generated in the secondary lithium battery, thereby increasing the thickness of the secondary lithium battery (J. Power Sources, 72 (1998), 66-70). Also, after charging the secondary lithium battery, when the secondary lithium battery is stored at a high temperature, the performance and stability of the secondary lithium battery may decrease.
In general, the capacity of an active material of an anode and a cathode can be increased in order to obtain the high capacity and high energy-density of a secondary lithium battery. Alternatively, a charging voltage of a secondary lithium battery can be set to be relatively high in order to extract a larger amount of capacity from the active material. Among such methods, when a charging voltage is set to be relatively high, the charging voltage is generally set to be equal to or greater than 4.3 V. However, at such a high voltage, the stability of a cathode active material may decrease, and a portion of an electrolyte may be oxidized, thereby forming a thick SEI membrane. Thus, the cycle characteristic of the secondary lithium battery may deteriorate.
For example, when the secondary lithium battery charged at a high voltage is stored at a high temperature, metal ions of the cathode active material are eluted to the electrolyte, and the eluted metal ions are deposited on the surface of the anode. Thus, the interfacial resistance of the secondary lithium battery increases, and accordingly the impedance of the secondary lithium battery increases, thereby reducing an open circuit voltage (OCV) of the secondary lithium battery. In this case, the discharge capacity of the secondary lithium battery may be remarkably decreased.
To overcome this problem, Japanese Patent Laid-open Publication No. 2006-286382 discloses that the cycle characteristic of a battery are improved by forming a lithium tetrafluoroborate (LiBF4) and vinylene carbonate membrane on a surface of an electrode. However, according to this method in which the formation of an appropriate membrane is induced by adding a small amount of organic material as an additive, the characteristics of an SEI membrane formed on a surface of an electrode vary according to the type of solvent used in an electrolytic solution or the electrochemical characteristics of the additive. The formed SEI membrane remains unstable and the solution decomposition reaction is not prevented when the above organic material is used, thereby reducing the performance of the battery.
Therefore, there is a need for an additive that can form a thinner and stronger membrane compared to conventional additives, in order to reduce the reactivity of an electrolyte, so that a secondary lithium battery using an electrolyte including the additive can have improved cycle and high temperature retention characteristics.